User demand for entertainment equipment is particularly high as a result of the rapid development of multimedia applications. Conventionally, the cathode ray tube (CRT) display, which is a type of monitor, is commonly used. However, the cathode ray tube display does not meet the needs of multimedia technology because of the large volume thereof. Therefore, many flat panel display techniques such as liquid crystal display (LCD) have been recently developed.
Conventional LCD panels have narrow view angles so the normal images displayed by them only can be viewed directly in front of the display area. If the display area is watched from an oblique view angle, color distortion occurs in what is viewed, and gray inversion may even occur. That is, what appears black is actually white and what appears white is actually black. Therefore, how to widen the view angle is an important subject for the LCD manufacturers.
Among various methods for widening the view angle, an LC vertical alignment (VA) technique is still one of the most popular techniques in the current LCD market. However, because liquid crystal molecules are aligned in the same direction (mono-domain vertical alignment), a normal image cannot be seen from a view angle perpendicular to or symmetric to the direction. No matter whether the liquid crystal molecules are realigned in a different direction after the electrical field existing therein changes, the view angle is also limited to the parallel direction of the liquid crystal molecules. Therefore, a multi-domain VA (MVA) technique has been put forth to improve the drawback of the prior art. However, according to MVA technology, additional ridges or bumps are required on the color filter to control the alignment of the tilt direction of liquid crystal molecules and automatically align tilt direction according to their region to which they belong. But because the existence of the bumps results in a need for precise alignment between a color filter and an active matrix substrate, and an additional over-coating must be formed on the color filter, the yield of this LCD panel is reduced and the cost thereof is increased. Therefore, bias-bending vertical alignment (BBVA) technology has been developed to solve the foregoing problem.
FIG. 1 is a schematic, cross-sectional diagram of a conventional LCD display panel of the bias-bending vertical alignment (BBVA) type. The LCD panel 100 comprises an upper transparent substrate 102, a lower transparent substrate 104 and a liquid crystal molecule layer 106 between the upper and lower substrates 102 and 104. A common electrode 108 is located over the upper transparent substrate 102. Pixel electrodes 110 are located over the lower transparent substrate 104. A main electric field exists between the common electrode 108 and the pixel electrode 110 to make liquid crystal molecules 116 have oblique positions. A control electrode 112 is formed on the lower substrate 104. An insulation layer 114 is interposed between the control electrode 112 and the pixel electrode 110. During operation, a bias voltage is generated first between the control electrode 112 and the common electrode 108. The bias voltage causes the corresponding liquid crystal molecule to be in a saturation state. Then, when another voltage is applied to the pixel electrode 110, the adjacent liquid crystal molecules are sequentially arranged in a predetermined orientation.
FIG. 2 is a conventional equivalent circuit diagram of a pixel region implanting BBVA technology. Three thin film transistors are located in each pixel region. The thin film transistors 202 and 204 serve as switches of the pixel electrode voltage (Vpixel). The thin film transistor 206 serve as a switch of the control electrode voltage (VCE). The three thin film transistors 202, 204 and 206 are controlled by the signals from the adjacent scan lines Gn-1 and Gn.
When the scan line Gn-1 is scanned, the thin film transistors 204 and 206 are on and the thin film transistor 202 is off. At this time, the control electrode voltage (VCE) and the pixel voltage (Vpixel) are written into the data lines Dm-1 and Dm through the thin film transistors 204 and 206, respectively. Therefore, a voltage difference is built between the control electrode voltage (VCE) and the pixel voltage (Vpixel). Next, when the scan line Gn is scanned, the thin film transistor 202 is on and the thin film transistors 204 and 206 are off. At this time, the voltage signal transmitted in the data line Dm is written into the pixel region through the thin film transistor 202. The control electrode voltage (VCE) is raised or dropped to a specific voltage by the pixel voltage (Vpixel) through a coupling effect generated by a capacitor C2.
However, three thin film transistors are used in the conventional circuit structure designed for BBVA technology, which reduces the aperture ratio. If one of the thin film transistors is damaged, the pixel is considered to be malfunctioning. On the other hand, too many thin film transistors connected to a same scanning line result in a severe RC delay on the scan signal.